nvmem: u-boot-env: use nvmem_add_one_cell() nvmem subsystem helper

nvmem: u-boot-env: use nvmem_add_one_cell() nvmem subsystem helper

[ Upstream commit 7c8979b42b1a9c5604f431ba804928e55919263c ]

Simplify adding NVMEM cells.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20231221173421.13737-3-zajec5@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Stable-dep-of: 8679e8b4a1eb ("nvmem: u-boot-env: error if NVMEM device is too small")
Signed-off-by: Sasha Levin <sashal@kernel.org>